The MET(metabolic equivalent of task) is are a unit of exercise intensity that indicates how many times more energy is consumed compared to when resting is 1.
Walking (4.8 km / h) is equivalent to 3 METs.
Wikipedia: Metabolic equivalent of task

Record of Average METs

Workouts measured in Zones are recorded with average METs.
It may not be recorded due to short exercise time.

The average METs are calculated by Apple Watch (watchOS).
The calculation method and the presence or absence of recording depend on your watchOS.
As far as we know, Apple has not disclosed how to calculate MET, but please enter your Health Profile correctly to improve measurement accuracy.

Display of Average METs

The average METs are stored in the workout record. (HKMetadataKeyAverageMETs)
Workouts measured with the Apple Workout app also record this value and may appear on Zones.